1. Field of the Invention
The present invention relates to cellular communications systems and, more particularly, to a frequency reuse plan therefor.
2. Description of the Related Art
Frequency reuse is the use of radio channels on the same carrier frequency for the coverage of geographically different areas and is necessary in order to construct practical, high-capacity cellular systems in traffic-dense areas, such as big cities. Needless to say, these geographically different areas which include the same radio carrier frequencies must be far enough apart to ensure that co-channel interference either does not arise or does not arise to an objectional level.
An important measurable characteristic of frequency reuse schemes is carrier-to-interference ratio ("C/I ratio"). The C/I ratio is defined to be the ratio of the level of the received desired signal to the level of the received undesired signal. Because of irregular terrain and the various shapes, types and numbers of local scatterers, the C/I ratio is dependent upon the instantaneous position of a mobile moving through a cell. Other factors such as antenna type, directivity and height, site elevations and positions, and the number of local interferers also affect the C/I ratio at various locations within a system.
The desired distribution of the C/I ratio in a system determines the number of frequency groups, F, which may be used. If the total allocation of N channels is partitioned into F groups, then each group will contain N/F channels. Since the total number of N channels is fixed, for example, there are 312 voice channels in the F.C.C. Standard A Band, a smaller number of F frequency groups would result in more channels per set and per cell site. Therefore, a reduction in the number of frequency groups would allow each site to carry more traffic, reducing the total number of sites needed for a given traffic load. However, decreasing the number of frequency groups and reducing the co-channel reuse distance results in a lower C/I distribution in the system.
FIGS. 1-3 are schematic illustrations of various prior art frequency reuse plans. In viewing these depictions, as well as all other illustrated frequency reuse plans, it should be appreciated that the illustrated cells are shown to have certain shapes. While in theory, cells may be envisioned as having any regular polygon shape, the important point is that the array of cells covers a plane without gaps or overlaps. Similarly, cell boundaries may be conceptually defined as lines at which the respective signal strengths of neighboring cells are equal. In reality, of course, because of such factors as random propagation effects, real cells only roughly approximate ideal cell shapes with ideal boundaries therebetween.
In considering the frequency reuse plans shown herein, it should be appreciated that equivalent site coverage areas, the same site locations, and 312 available voice channels are assumed in each plan. This reflects reality and allows fair comparisons to be made among the various plans.
Referring now to FIG. 1, there is shown a diagram of a 7/21 Cloverleaf Cell Plan as has heretofore been implemented by Ericsson, the assignee of the present invention. If may be seen that this plan employs a number of sites 2, each site serving three cells (or "sectors") 4. Each cell 4 contains a dedicated antenna system, a control channel, a signal strength receiver, and voice channels. In FIG. 1, certain groups of co-channel cells, i.e., cells employing the same frequencies, are shown cross-hatched, e.g., cells 4A and 4B. Using the same terminology, sites 2A and 2B should be appreciated to be co-channel sites.
Further with respect to FIG. 1, it may be seen that site 2A is centrally located within the illustrated system and that the outlying sites are all shifted two (2) units in a first ("i.sup.t h") direction and one (1) unit in a second ("j.sup.th ") direction from the central site. Defining i and j as shift parameters, the illustrated plan may be considered to have shift parameters of two (2) and one (1) respectively. Shift parameters are important characteristics of frequency reuse plans and will therefore be discussed with respect to each plan described herein.
Recalling that there are 312 available voice channels in the F.C.C. Standard A Band, the 7/21 Cloverleaf Cell Plan shown in FIG. 1 uses some 21 frequency groups in its seven repeating sites with approximately (although averaging somewhat less) 15 channels per group.
Supporting the three cell configurations shown in FIG. 1, each cell has antenna pointing azimuths separated by some 120.degree.. More specifically, in practice, each cell uses 60.degree. transmit antennas and two (2) 60.degree. diversity receive antennas with the same pointing azimuths.
Referring now to FIG. 2, shown therein is a Three Rhomb Cell Plan as has heretofore been implemented by numerous operators, including the companies of the Bell System. Sites, cells and shift paraments are similarly marked in FIG. 2 as they were in FIG. 1. Examining FIG. 2, it should be appreciated that the Bell 7/21 Three Rhomb Cell Plan uses 21 Frequency groups in a seven site reuse pattern with approximately 15 channels per group. As with the Ericsson 7/21 Cloverleaf Cell Plan, the shift parameters, i and j, are two (2) and one (1) respectively. Site geometry in the Bell plan involves three cells 4 or sectors at each site 2. The antenna pointing azimuths of each cell are separated by 120.degree.. Each cell uses 120.degree. transmit antennas and two (2) 120.degree. diversity receive antennas with the same pointing azimuths. Additionally, each cell is approximated by the shape of a rhomboid.
Referring now to FIG. 3, shown therein is a 4/24 Six Triangle Cell Plan as has heretofore been implemented by Motorola. This plan uses 24 frequency groups in a four site reuse pattern with 13 channels per group. The corresponding shift parameters i and j are two (2) and zero (0), respectively. The site geometry involves six cells 4 at each site 2 with antenna pointing azimuths separated by 60.degree.. Each cell uses one (1) 60.degree. antenna with the transmit and receive functions duplexed. Additionally, each cell is approximated by the shape of an equilateral triangle.
Further details regarding each of the above plans will be set forth below in various comparisons with the plan according to the present invention. In general however, it may be noted and should be appreciated by those skilled in the art that each of the prior art systems possesses shortcomings in that a number of important system characteristics; e.g., C/I performance, capacity, utilization, and site position tolerance, could be improved.